Process for producing semipermeable membrane

ABSTRACT

A process for producing a semipermeable membrane having a high flux, excellent in solute rejection and useful for reverse osmosis, ultrafiltration and the like which comprises dipping a semipermeable membrane of acrylonitrile polymer containing 40-100% by mole of acrylonitrile in a solution of an amine and/or an alkali, or casting a solution of said polymer containing an amine and/or an alkali to prepare a semipermeable membrane, and optionally, treating the semipermeable membrane thus obtained with plasma.

The present invention relates to a process for improving semipermeablemembranes comprising an acrylo-nitrile polymer used for reverse osmosis,ultrafiltration or the like. More particularly, it relates to a processfor improving semipermeable membranes so as to increase waterpermeability (hereinafter, referred to as "flux") without damaging thesolute rejection of the membranes.

Recently, a reverse osmosis or ultrafiltration process usingsemipermeable membranes made of cellulose acetate, polyamide or the likeis widely used in various fields such as production of fresh water fromsea water, treatment of waste water, control of electrodeposition bath,the food industry and the medical industry. What is important in theseseparation processes using membranes is the selective permeability ofthe membranes, i.e. solute rejection and flux. Particularly, it is nottoo much to say that the quantity of flux is a factor determining theeconomic value of the processes.

To date, a large number of studies have been made to increase the fluxof semipermeable membranes used for reverse osmosis and ultrafiltration,and even the studies reported in patents and papers are too numerous toenumerate. Many of the studies are closely related to a membrane castingprocess, so that it may be said that the studies to increase the flux ofsemipermeable membranes are equivalent to a study of said process.

Of these studies, typical ones may be a study for producing asymmetricmembranes having active surface skin layers of the smallest possiblethickness, a study for producing an ultrathin membrane corresponding tothe active surface skin layer followed by backing the membrane on aporous support into a composite, and the like. Although these studiesmay be a regular approach to the improvement of flux, it is not alwaystrue that they provide a general and more practical method.

On the other hand, it is well known that acrylonitrile copolymers are araw material for the production of semipermeable membranes. Althoughmany trials have been made to produce semipermeable membranes ofacrylonitrile since the process disclosed in Japanese Patent Kokai(Laid-Open) No. 6,257/72, the membranes produced from acrylonitrilepolymers are low in solute rejection in spite of their large flux asmentioned in Japanese Patent Kokai (Laid-Open) No. 3,971,75.Particularly when the solute is sodium chloride which is a low molecularweight electrolyte, they are said to exhibit an extraordinarily lowrejection.

This is the reason why semipermeable membranes of acrylonitrile polymercannot be used for reverse osmosis, even though they may find apractical application in ultrafiltration.

Previously, in view of the good film forming property, thermalresistance, acid resistance and alkali resistance of acrylonitrilepolymers, the inventors extensively studied a process for producingsemipermeable membranes capable of rejecting even low molecularelectrolytes such as sodium chloride at a high rate. As a result, theinventors succeeded in producing semipermeable membranes ofacrylonitrile polymer having better thermal, mechanical and chemicalproperties and higher solute rejection than those of the conventionalcellulose acetate membranes when used for reverse osmosis, by theepoch-making process of plasma-treatment of porous membranes ofacrylonitrile polymer (Japanese Patent Publication No. 38,988/77). Thisprocess brought about a great advance never attainable by the prior artinto the field of semipermeable membranes of acrylonitrile polymer.

The inventors continued a further study to improve the performances ofsemipermeable membranes of various acrylonitrile polymers includingplasma-treated ones. As the result, the inventors found that thesesemipermeable membranes of acrylonitrile polymers can remarkably beimproved in the flux without largely damaging their solute rejection bydipping them in a solution of an amine and/or an alkali, or by treatingthem with plasma after the dipping treatment, or, when semipermeablemembranes of acrylonitrile polymer are produced by wet casting process,by adding an amine and/or an alkali to the solution to be cast or bytreating the semipermeable membrane thus obtained with plasma. Thepresent invention is based on this finding.

It is the object of the present invention to provide a process forproducing semipermeable membranes excellent in solute rejection and highin flux by dipping a semipermeable membrane of acrylonitrile polymer ina solution of an amine and/or an alkali, or by thereafter treating themembrane with plasma, or, when a semipermeable membrane of acrylonitrilepolymer is produced by wet casting process, by adding an amine and/or analkali to the solution to be cast or by treating the semipermeablemembrane thus obtained with plasma.

It is well known that, if an acrylonitrile polymer is dissolved inN,N-dimethylformamide, mixed with an amine or an alkali and then leftstanding, it is colored brown. The colored substance is considered toappear due to the formation of naphthylidine ring. It is consideredthat, in the process of the invention, naphthylidine ring is similarlyformed by carrying out the same treatment or by dipping a semipermeablemembrane of acrylonitrile polymer in a solution of amine or alkali. Thisformation of naphthylidine ring is believed to provide the improvementof flux without damaging solute rejection.

Hereunder, the embodiment of the present invention will be illustratedin detail.

The acrylonitrile polymer constituting the semipermeable membranes usedin the present invention includes polyacrylonitrile and various types ofcopolymers containing acrylonitrile monomer as a component, both ofwhich can be produced by the well-known methods. As comonomersconstituting the copolymers together with acrylonitrile, there can bementioned various well-known nonionic and ionic monomers copolymerizablewith acrylonitrile. For example, the nonionic monomers includeacrylamide, diacetone acrylamide, N-vinyl-2-pyrrolidone, hydroxyethylmethacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, methylmethacrylate, ethyl methacrylate, vinyl acetate, vinyl chloride, styreneand the like. The ionic monomers include acrylic acid, methacrylic acid,ethylenesulfonic acid, methallylsulfonic acid, sulfopropyl methacrylate,vinylbenzenesulfonic acid; methallic salts thereof; tertiary amines suchas 2-vinylpyridine, 4-vinylpyridine and dimethylaminoethyl methacrylate;and salts of quaternary amines obtainable by alkylating the tertiaryamines.

As referred to in the present invention, the term "acrylonitrilepolymer" means a copolymer comprising at least 40% by mole ofacrylonitrile and 60-0% by mole of at least one comonomber mentionedabove. If the percentage of acrylonitrile is less than 40% by mole, theeffect of the invention cannot be exhibited, so that practical value isnot recognized.

Accordingly, as the acrylonitrile polymer usable in the invention,acrylonitrile copolymers containing 40% by mole or more of acrylonitrileare preferable, and those containing 70-95% by mole of acrylonitrile areparticularly preferable.

Molecular weight of the polymers is preferably 5,000-5,000,000 from theviewpoint of sufficiency of membrane mechanical strength.

When a semipermeable membrane of acrylonitrile polymer is dipped in asolution of amine and/or alkali in the process of the invention, thesemipermeable membrane of acrylonitrile polymer is not particularlylimited in the process of production. However, particularly preferablemembranes are a semipermeable membrane produced by wet casting processcomprising usual casting process of solution, partial evaporation ofsolvent and gelation and a semipermeable membrane obtained by furthertreating, with hot water, the porous membrane obtained by wet castingprocess.

These semipermeable membranes have various shapes such as flat film,tube, hollow fiber, string-form, composite with other porous support andso on, which all can be used in the process of the invention.

One example of the wet casting process preferably used in the inventionis as follows. First of all, polyacrylonitrile or an acrylonitrilecopolymer is dissolved into solvent only or a mixture of solvent andadditives so that the polymer concentration reaches 5-30% by weight.When amine or alkali is to be added, they are then added and dissolved.

As the solvent, an aqueous solution of inorganic salt or a polar organicsolvent such as N,N-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide and the like is used.

As the additive, polyols such as polyethylene glycol, polypropyleneglycol and the like are used, among which polyethylene glycol having anaverage molecular weight of 100-2,000 is particularly preferable. It isindispensable that the amount of said additive falls in the range wherethe additive is mutually soluble with the polymer solution. Preferableamount of said additive is 5-20% by weight based on the total weight ofpolymer solution.

Subsequently, the polymer solution is cast on a glass plate with adoctor blade.

Temperature of the casting may be a temperature where the casting ispossible. Preferable range of casting temperature is 10° C. to 80° C.

Thickness of the cast solution has a relation with thickness ofsemipermeable membrane.

Accordingly, thickness of cast solution is usually controlled so as toform a semipermeable membrane having a thickness of about 100μ. The castsolution is dipped into non-solvent either immediately or afterevaporating the solvent at the surface of solution for a while.

Preferably, the time period of evaporating the solvent is 0-60 minutes,and temperature of the evaporation is the range from 0° C. to boilingpoint of the solvent. Then, the cast solution from which the solvent atsurface has been partially evaporated or the cast solutoin not subjectedto evaporation process at all is dipped in nonsolvent and made to gel.As the nonsolvent, water or a mixture of water and organic solvent isused.

The amine used in the invention is not critical, but all the substancesextensively known as amine can be used. As used in the invention, theterm "amines" means a series of compounds shown below which includequaternary ammonium salts, too. Thus, for example, they includecompounds represented by the following general formula: ##STR1## whereinR₁, R₂ and R₃ are H, C₁₋₁₈ alkyl group, phenyl group, aralkyl group orcyclohexyl group, compounds represented by the following generalformula: ##STR2## wherein R₁, R₂, R₃ and R₄ are H, C₁₋₁₈ alkyl group,phenyl group, aralkyl group or cyclohexyl group, and X is halogen, aswell as alkanolamines, hydroxylamines and the like.

Concrete examples of said amine include ammonia, methylamine,dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine,n-propylamine, iso-propylamine, n-butylamine, di-n-butylamine,tri-n-butylamine, iso-butylamine, di-iso-butylamine, tri-iso-butylamine,t-butylamine, di-t-butylamine, tri-t-butylamine, laurylamine,stearylamine, aniline, diphenylamine, methylaniline, cyclohexylamine,their quaternary salts with inorganic acids, their quaternary salts withalkyl halide, ethanolamine, diethanolamine, triethanolamine and thelike. Among them, preferable amines are dimethylamine, trimethylamine,diethylamine, triethylamine, n-propylamine, n-butylamine,di-n-butylamine, tri-n-butylamine, iso-butylamine, di-iso-butylamine,tri-iso-butylamine, t-butylamine, di-t-butylamine, tri-t-butylamine,ethanolamine, diethanolamine, triethanolamine and the like.

Though the alkali usable in the invention is not critical, hydroxides ofalkali metals are preferable. Concretely, sodium hydroxide, potassiumhydroxide and the like can be mentioned.

When a semipermeable membrane of acrylonitrile polymer is dipped in asolution of amine and/or alkali in the invention, liquid amines may beused as they are. When the amines are used in the form of a solution,concentration of the amines is not critical, but a concentration atwhich the membrane of acrylonitrile polymer is colored may be used,though the concentration may be dependent also on the kind of amine oralkali. Solvent for preparing the solution is not critical, thoughwater, alcohols and ketones are preferable. Particularly preferablesolvents for this purpose are water, methanol, ethanol, propanol,acetone and methyl ethyl ketone.

Temperature, time period and pressure of the dipping treatment are notcritical.

When an amine and/or an alkali is added to the casting solutioncontaining acrylonitrile polymer of the invention, the amount of amineand/or alkali may be an amount enough to form a naphthylridine ring inthe acrylonitrile polymer, though it may be dependent on the kind ofamine or alkali, too. Concretely saying, its amount is in the range of0.01-20% by weight and preferably in the range of 0.01-10% by weightbased on the weight of acrylonitrile polymer solution, i.e. the totalweight of acrylonitrile polymer and solvent.

As the conditions for the plasma treatment of semipermeable membrane,the conditions mentioned in Japanese Patent Publication No. 38,988/77may be employed, for example. Specifically, a gas such as hydrogen,helium, argon, nitrogen, oxygen, carbon monoxide, carbon dioxide,ammonia or the like is introduced into such an evacuated vessel as shownin FIG. 1 of the gasette of the above-mentioned patent so that pressurein the vessel reaches 0.01-10 Torr and then a voltage of 0.5-50 KV of analternating or direct current is applied between the electrodes, wherebya plasma is generated. The time period of the treatment is preferably inthe range of 5 seconds to 600 minutes.

As has been mentioned above, the characteristic feature of the presentinvention consists in that flux of semipermeable membrane can beincreased without damaging its solute permeability by dipping asemipermeable membrane of acrylonitrile polymer in a solution of anamine and/or an alkali, or by treating the membrane with plasma afterthe dipping treatment, or, when a semipermeable membrane ofacrylonitrile polymer is produced by wet casting process, by adding anamine and/or an alkali to the casting solution or by treating thesemipermeable membrane thus obtained with plasma.

The semipermeable membrane obtained by the process of the presentinvention is remarkably increased in flux as compared with hithertoknown acrylonitrile polymer type semipermeable membranes withoutdamaging solute rejection. Therefore, it has a great economical effectand is practically usable in various industries.

That is, the semipermeable membranes obtained by the process of thepresent invention can extensively be used for separation andconcentration of substances by reverse osmosis, ultrafiltration or thelike, and more concretely for production of fresh water from sea water,treatment of waste water, concentration of fruit juices, separation ofnonaqueous fluids and other processes.

Further, the effect of the present invention can also be exhibited bycombining the present invention with other treatments such as thedipping in aqueous solution of inorganic salt mentioned in JapanesePatent Kokai (Laid-Open) No. 123,583/79, or the dipping treatment inprotic acid mentioned in Japanese Patent Kokai (Laid-Open) No.130,482/79, or the treatment with hot water mentioned in Japanese PatentApplication No. 155,919/78, or other inventions. Such combinations arealso involved in the scope of the present invention.

The present invention will be illustrated more concretely with referenceto the following examples which are presented in no limitative way.

In the examples, solute rejection is a quantity defined by the followingequation:

EXAMPLE 1

A copolymer consisting of 89% by mole of acrylonitrile and 11% by moleof methyl acrylate was synthesized according to the known process. Afterdissolving 20 parts of the copolymer into a solvent mixture consistingof 70 parts of N,N-dimethylformamide and 10 parts of foramide, theresulting solution was cast on a glass plate heated to 40° C. so thatthickness of the solution reached 250μ. After an evaporating period ofone minute, the glass plate was thrown into water kept at 10° C. for thesake of gelation. Two hours later, the membrane released from the glassplate was taken out and dried at room temperature for 24 hours.Thickness of the membrane was 100μ. Its bubbling point was 32 kg/cm².After dipping the membrane in amine or alkali under the treatingconditions shown in Table 1 and then washing it with water, it was driedat room temperature for 24 hours. Then, it was treated with plasmaaccording to the method mentioned in Japanese Patent Publication No.38,988/77, i.e. in a plasma treatment apparatus made from a bell jarhaving one pair of electrodes inside, to produce a plasma-treatedmembrane.

Conditions of the plasma treatment were as follows:

    ______________________________________                                        Gas                   Helium                                                  Degree of vacuum      0.2 Torr                                                Discharge voltage     3 KV                                                    Discharge current     25 mA                                                   Treating time         40 minutes.                                             ______________________________________                                    

Then, the membrane as attached to a circulation type of reverse osmosisapparatus (effective membrane area: 13.0 cm²) conventionally used inlaboratories, and tested on permeation of aqueous sodium chloridesolution (0.5% by weight). Thus, under a pressure of 50 kg/cm², anaqueous sodium chloride solution having a temperature of 25° C. wassupplied to the cell at a rate of 630 ml/minute, and flux and saltrejection were measured 24 hours after the start of experiment. Theresults obtained were as shown in Table 1. An increase in flux wasproduced by the dipping treatement in amine or alkali.

                                      TABLE 1                                     __________________________________________________________________________    Treatment with amine or alkali                                                                           Temperature                                                                          Time of   Salt                                                 Concentration                                                                         of treatment                                                                         treatment                                                                          Flux rejection                         Kind        Solvent                                                                              (% by wt.)                                                                            (°C.)                                                                         (hr) (LMH).sup.1                                                                        (%)                               __________________________________________________________________________    1     --    --     --      --     --    4.6 97.1                              2 Monoethanolamine                                                                        Methanol                                                                             20      45      6   10.3 97.8                              3 Triethylamine                                                                           Methanol                                                                             10      45     20   11.5 97.6                              4 Triethylamine                                                                           Methanol                                                                             10      45     30   12.1 97.7                              5 n-Butylamine                                                                            Ethanol                                                                              15      45     10   11.8 98.0                              6 Cyclohexylamine                                                                         Methanol                                                                             10      30     10   10.7 97.3                              7 Diphenylamine                                                                           Isopropanol                                                                          10      45     24    9.6 96.8                              8 Potassium hydroxide                                                                     Methanol                                                                             0.7     45     10   10.3 98.1                              9 Sodium hydroxide                                                                        Methanol                                                                             0.005   45     10   10.7 98.0                              __________________________________________________________________________     .sup.1 LMH = liter/m.sup.2 · hr                                 

EXAMPLE 2

A copolymer consisting of 90% by mole of acrylonitrile and 10% by moleof vinyl acetate was synthesized by the well-known process.

After dissolving 21 parts of the copolymer into a solvent mixtureconsisting of 69 parts of N,N-dimethylformamide and 10 parts offormamide, the resulting solution was cast on a glass plate at a roomtemperature of 25° C. so that thickness of the cast solution reached250μ. After evaporation for one minute, the glass plate was thrown intowater kept at 17° C. for the sake of gelation. Two hours later, themembrane released from glass plate was taken out and dried at roomtemperature for 24 hours. Bubbling point of the membrane was 35 kg/cm².After dipping the membrane in amine or alkali under the conditions shownin Table 2, it was washed with water and dried at room temperature.Thickness of the membrane was 100μ.

The membranes thus obtained were tested for salt rejection in the samemanner as in Example 1. The results are shown in Table 2, whichdemonstrate that flux of membranes increases when the membranes aresubjected to dipping treatment with amine or alkali.

                                      TABLE 2                                     __________________________________________________________________________    Treatment with amine or alkali                                                                           Temperature                                                                          Time of   Salt                                                 Concentration                                                                         of treatment                                                                         treatment                                                                          Flux rejection                         Kind        Solvent                                                                              (% by wt.)                                                                            (°C.)                                                                         (hr) (LMH)                                                                              (%)                               __________________________________________________________________________    1     --    --     --      --     --    4.0 0                                 2 Monoethanolamine                                                                        Methanol                                                                             20      25     48   12.6 6.0                               3 Potassium hydroxide                                                                     Methanol                                                                             0.6     45     20   11.8 7.2                               4 Triethylamine                                                                           Methanol                                                                             20      45     20   12.0 6.8                               __________________________________________________________________________

EXAMPLE 3

A copolymer consisting of 89% by mole of acrylonitrile and 11% by moleof methyl acrylate was synthesized by the well-known process. Afterdissolving 20 parts of the copolymer into a solvent mixture consistingof 70 parts of N,N-dimethylformamide and 10 parts of formamide, an amineor an alkali was added as shown in Table 3 and the resulting mixture wasallowed to stand at 80° C. for 4 hours. While keeping the temperature,the solution was cast on a glass plate heated to 40° C. so thatthickness of the cast solution reached 250μ. After evaporation for 11minutes, the glass plate was thrown into water kept at 10° C. for thesake of gelation. Two hours later, the membrane released from the glassplate was taken out and dried at room temperature for 24 hours.Thickness of the membrane was 100μ. Then, a plasma-treated membrane wasprepared under the same plasma-treatment conditions as in Example 1. Themembranes were tested for salt rejection in the same manner as inExample 1. The results are shown in Table 3. Flux was increased by theaddition of amine or alkali.

                  TABLE 3                                                         ______________________________________                                                                       Salt                                                                          rejec-                                         Amine or alkali added  Flux*.sup.2                                                                           tion                                           Kind           Amount*.sup.1                                                                             (LMH)   (%)                                        ______________________________________                                        1      --          --           4.6  97.1                                     2    Monoethanolamine                                                                            0.5         10.5  97.7                                     3    Triethylamine 0.3         11.6  96.8                                     4    Triethylamine 0.4         12.1  97.5                                     5    n-Butylamine  0.2         11.3  98.0                                     6    Cyclohexylamine                                                                             1.0         12.4  98.1                                     7    Diphenylamine 2.0         10.2  97.3                                     8    Potassium hydroxide                                                                         1.0         11.8  97.9                                     9    Sodium hydroxide                                                                            1.0         11.2  97.7                                     ______________________________________                                         *.sup.1 Percent by weight based on (acrylonitrilemethyl acrylate copolyme     + dimethylformamide + formamide)                                              *.sup.2 LMH = liter/m.sup.2 · hr                                

EXAMPLE 4

A copolymer consisting of 90% by mole of acrylonitrile and 10% by moleof vinyl acetate was synthesized by the well-known process.

After dissolving 21 parts of the copolymer into a solvent mixtureconsisting of 69 parts of N,N-dimethylformamide and 10 parts offormamide, amine or alkali was added as shown in Table 4, and theresulting mixture was allowed to stand at 80° C. for 4 hours. Whilekeeping the temperature, the solution was cast on a glass plate heatedto 40° C. so that thickness of the cast solution reached 250μ. Afterevaporation for one minute, the glass plate was thrown into water keptat 17° C. for the sake of gelation. Two hours later, the membranereleased from the glass plate was taken out and dried at roomtemperature for 24 hours. Thickness of the membrane was 100μ.

The membranes thus obtained were tested for salt rejection in the samemanner as in Example 1. The results are shown in Table 1. Flux ofmembrane is increased by the addition of amine or alkali.

                  TABLE 4                                                         ______________________________________                                                                      Salt                                            Amine or alkali added Flux    rejection                                       Kind           Amount     (LMH)   (%)                                         ______________________________________                                        1      --          --          4.0  0                                         2    Monoethanolamine                                                                            0.5        11.7  5.8                                       3    Triethylamine 0.3        13.1  5.4                                       4    Potassium hydroxide                                                                         0.2        12.5  6.7                                       ______________________________________                                    

EXAMPLE 5

A membrane was prepared in the same manner as in Example 4. It wasattached to a circulation type of reverse osmosis apparatus (effectivemembrane area: 13.0 cm²) usually employed in laboratories, and testedfor permeation of 0.5% (by weight) aqueous solution of potassiumferricyanide. Thus, under a pressure of 50 kg/cm², an aqueous solutionof potassium ferricyanide kept at 25° C. was supplied to the cell at arate of 630 ml/minute, and flux and salt rejection were measured 24hours after the start of experiment. The results obtained were as shownin Table 5. It is known from the table that flux of membrane isincreased by the dipping treatment with amine or alkali.

                  TABLE 5                                                         ______________________________________                                                                      Salt                                            Amine or alkali added Flux    rejection                                       Kind          Amount*.sup.1                                                                             (LMH)   (%)                                         ______________________________________                                        1     --          --           3.9  94.3                                      2   Monoethanolamine                                                                            0.5         11.2  97.6                                      3   Triethylamine 0.3         12.7  97.2                                      4   Potassium hydroxide                                                                         0.2         11.8  98.1                                      ______________________________________                                         *.sup.1 The same as in Table 4.                                          

What is claimed is:
 1. A process for producing a semipermeable membranewhich comprises dipping a semipermeable membrane of acrylonitrilepolymer containing 40-100% by mole of acrylonitrile in a solution ofsodium hydroxide or potassium hydroxide and thereafter treating it withplasma.
 2. A process for producing a semipermeable membrane whichcomprises using, as casting solution, a solution containing sodiumhydroxide or potassium hydroxide when a semipermeable membrane ofacrylonitrile polymer containing 40-100% by mole of acrylonitrile isproduced by wet casting process, and then subjecting the resultingsemipermeable membrane to a plasma treatment.